The results come a year after the horse meat scandal which led to products such as burgers and lasagne being removed from supermarket shelves after they were found to contain traces of horse.

According to a presentation by NTU lead scientist Professor Ellen Billett, seen by trade magazine The Grocer, the undeclared offal was detected at a level of 1% or more, with some products containing more than one type of offal.

The report said the five products with undeclared blood contained serum "at a much higher level than expected".

In light of the research, the Food Standards Agency (FSA) and the Department for Environment, Food and Rural Affairs (Defra) is working with NTU to develop validated testing methods.

The results of this work are due to be published in late summer.

A Defra spokeswoman said it was not possible to draw conclusions from the study until there was "a robust method that is technically sound" in place.

A spokesman for the FSA said: "When businesses add offal to their food products then they must declare it on the label to inform consumers.

"Local authorities will take follow up action where undeclared offal is found, to investigate the reasons for its presence and take further enforcement action as necessary."

Trade magazine The Grocer, which initially published the university's findings, noted that the survey was small and involved new, unvalidated testing methods, and the retailers and suppliers involved had therefore not been named.

Julia Glotz, buying and supplying editor of The Grocer, said: "It is clearly concerning that, a year after Horsegate, we could once again be looking at issues with meat mislabelling.

"The key question now is to what extent the findings from the East Midlands reflect national trends.

"As researchers used new testing methodologies, we will need to wait until work to develop validated tests has been completed later this summer. But then we urgently need to conduct wider tests to determine how big a problem undeclared use of offal and added blood really is across the UK.

"Some people might say there's nothing wrong with a bit of offal or blood, but the point is they should be declared clearly as an ingredient on the label so consumers can make an informed choice.

"Consumers rightly expect food labels to be honest. Any suggestion that labels aren't a true reflection of what's in the pack could seriously undermine consumer trust in meat products once more."